It is known to use accurately maintained broadcast, cellular and navigation signals for providing a timing reference. Example signals are frequency standards and broadcast radio clock signals, cellular base stations, and Loran.
Typically, the signals are:                Designed specifically for providing a time or frequency reference (for example frequency standard transmitters);        Designed for navigation purposes (for example eLoran transmitters);        Designed for some other purpose, but requiring accurate timekeeping for that other purpose (for example CDMA cellular base stations); or        Designed for some other purpose, but implemented for convenience to have accurate timekeeping (for example broadcast transmitters).        
It has been observed that some signals designed for some other purpose—so called “signals of opportunity”—are very stable in their timekeeping, and indeed some are substantially synchronised to GPS. Their transmitter-location is normally also fixed.
The timing of many of these signals of opportunity may be reliable and known at the transmitter (or at a reference station listening to the transmitter). This may mean, for example, that a given portion of the signal is transmitted by the transmitter at a precisely known time instant, relative to the reference clock.
For low accuracy applications, the timing at the receiver can be taken to be that of the transmitter. That is, a receiver can assume that the time of arrival of the signal, which it observes, is identical to the precisely known time instant of transmission at the transmitter. This approximation ignores the propagation delay between the transmitter and the receiver. The advantage of ignoring propagation delay is that the time can be estimated even if the current device location is unknown.
For some applications this low-accuracy approach is sufficient—the time-error produced by the propagation of the signal from the transmitter to the user's receiver (and optionally to the reference station) is so small that it can safely be ignored.
For precision-timing applications, where reliability is important, the foundation time reference may be a Global Navigation Satellite System (GNSS) such as the Global Positioning System (GPS), and the signal of opportunity may be a back-up. In these applications, the receiver device location is fixed, and the secondary signal, the signal of opportunity is monitored to maintain accurate timing operation, when needed. In this case, the secondary signal is calibrated at the receiver—in the location where it is used—by comparing it with the foundation time reference (e.g. GPS) in normal operation. For example, the time of arrival of a portion of the signal of opportunity could be determined precisely according to GPS time, when GPS signals are available. This will establish a fixed offset relationship between GPS time and the arrival times of subsequent portions of the signal of opportunity, because the signal of opportunity is synchronised to GPS time. Even if GPS reception is lost, the device can still maintain accurate timing, by detecting the times of arrival of subsequent portions of the signal of opportunity. This can produce accurate results as long as the device location does not change.